Proximity electron tunneling spectroscopy I. Experiments on Nb

Abstract

A quantitative proximity electron tunneling spectroscopy (PETS) is demonstrated for the study of strong coupling superconductors which do not form suitable insulating oxides for conventional McMillan-Rowell tunneling spectroscopy. Proximity junctions of the form C-AI203-AI/ S are employed, with A1 thickness d~r <-100 A. Here S is the superconductor of interest and C is any convenient counterelectrode. The physical basis for the method, experimental techniques, and data obtained from foils of Nb are presented. The results for Nb include the energy-dependent pair potential As(E), the renormalization function Z(E), effective phonon spectrum a 2F(to), electron-phonon coupling constant A, and Coulomb pseudopotential lz *. A full discussion of the underlying theory and details of the methods of analysis employed to obtain, in addition, the pair potential of the Al proximity layer are contained in a following paper.

Structure and degeneracy of vortex lattice domains in pure superconducting niobium: A small-angle neutron scattering study

“…21 Tunneling measurements were also noted to be very sensitive to oxidation of Nb at the interface. 22 With high-purity samples of resistance ratios R 275 K / R 9.3 K տ 1000, further investigations revealed no evidence of a second gap in ultrasonic attenuation 23 and thermodynamic measurements. 24 Furthermore the temperature dependence of the specific heat in the superconducting state from the last mentioned study 24 is consistent with a relatively isotropic single energy gap.…”

Proximity electron tunneling spectroscopy. IV. Electron-phonon coupling and superconductivity of tantalum

“…The foils were usually of 50 gm initial thickness, reduced somewhat by electropolishing before mounting in the vacuum chamber. 2 The final state of outgassing for Ta was typically 5 min at -2600°C and p<5x 10-gTorr. Inspection of the foil after recrystallization typically reveals single-crystal domains of size -0.5 mm with pronounced grain boundaries.…”

“Break-junction” technique in application to layered superconductors (Review Article)

“…By the time the scanning tunneling microscope (STM) was invented [9,10]., classic SIS and NIS contacts (S is a superconductor, N is a non-ballistic layer of normal metal, I is the insulator) were usually constructed using a natural (oxidation) or artificial (mesa structure) oxide layer between two superconducting electrodes, as well as a vacuum barrier [11][12][13]. Along with the likes of "solid" tunnel junctions, squeezable contacts [14][15][16][17][18] also enjoyed a lot of popularity, in which the role of the tunnel barrier was played by a gap with a thickness about equal to the interatomic distance between two tightly pressed superconducting electrodes made of the studied material, immersed in liquid helium.…”